How many gph pump do I need

If you go by the suggested 5 turnovers per hour rule, you'd pick at least a 750 gph pump.
But pumping 3 tanks high with room for maintenance should put a bit of head loss on the pump reducing that 750gph some.
It also really depends on the type fish you are keeping.
If you want fish such as many anabantids or discus, that like slow moving water, something a little less powerful may be sufficient.
If you keep riverine cichlids such as Tomocichla, or fish like African Tiger fish or South American Hoplius aimara that live in rapids or are from crosscurrent areas under waterfalls, you need the most power you can afford.